This invention relates to a method for making improved tubular metallic shafts for golf clubs and other sporting implements.
As is commonly known, a golf shaft undergoes a significant stress during a golf swing at the portion of the shaft where the club head is attached, that is, at the tip portion. Typically, this tip portion of the shaft is of the narrowest diameter with respect to the remainder of the shaft since most golf shafts have a tapered configuration. Thus, unless this tip portion has a sufficient tip strength, it is especially susceptible to deformation when excessive force is used in hitting a golf ball or, in the alternative, a mis-hit occurs and the club head hits the ground. Tip strength is defined as the weight necessary to cause permanent deformation in the shaft when hung about 20 inches from the tip area.
The most convenient way of eliminating such a problem area on the shaft would be to increase its diameter to a value closer to the diameter of the rest of the shaft. Such a remedy is highly undesirable, however, because the weight distribution and moment of inertia inherent in a narrowing diameter or tapering shaft is necessary for execution of the most effective golf swing. More particularly, a tapered shaft is necessary in order to provide the proper "flex" and "flex point" of the shaft for an effective stroke. Both the "flex" and the "flex point" are determined according to the tapering nature of the shaft. In addition, it is undesirable to increase the diameter of the shaft because the hosel of commonly used club heads will not accommodate a larger diameter shaft.
Consequently, various techniques have been used to both reinforce this segment of the shaft (and increase tip strength) and to maintain a narrowing characteristic on the shaft. The most common technique is perhaps the incorporation of a reinforcing metal insert. Such an insert, however, adds undesired weight to the shaft and also requires a retaining feature such as a retaining pin or a special mechanical joining operation to hold the insert in place. Thus, it is desirable to design a shaft without the excessive weight resulting from the use of an insert and having a wall thickness along the tapered length and the tip portion of the shaft able to provide a desired weight distribution and to withstand the forces exerted on the shaft tip.
Methods for making shafts with varying wall thickness are contemplated in the prior art. For example, U.S. Pat. No. 2,095,563 to Cowdery discloses a method of making a golf shaft wherein the shaft is formed to have an OD of several steps by a plurality of draws while an internal mandrel controls the wall thickness at each step.
U.S. Pat. No. 2,240,456 to Darner and U.S. Pat. No. 4,616,500 to Alexoff show methods for providing varying wall thickness on a shaft with a constant outer diameter.
U.S. Pat. No. 3,292,414 to Goeke shows a method that provides a shaft with a tapered end, the tapered end having internal corrugations for strengthening.
U.S. Pat. No. 3,841,130 to Scott, Jr. et al. shows a baseball bat with a tapered, constant-thickness wall.
The shafts disclosed in these patents, however, do not provide a sufficiently strong shaft tip while also providing an optimal moment of inertia for use, for example, as a golf shaft.